Embryonic Period Flashcards
What are the steps of gastrulation?
Appearance of primitive streak on the dorsal surface of the epiblast (end of 2nd week)
Cells migrate through streak laterally and towards the head (cranial end) (not complete in caudal region until week 4)
Cranial —–> Caudal
Epiblasts, migrate, and invaginate and displace the hypoblast; forming the three germ layers (all derived from epiblast)
Define gastrulation.
Process establishing the three germ layers
Formation of the trilaminar disc
Sets the axes observed in the adult
What are the three germ layers? Which structures are formed from each one?
Ectoderm = organs & structures that maintain contact with the outside world
e.g. nervous system, epidermis
Mesoderm = supporting tissues
e.g. muscle, cartilage, bone, vascular system (inc. heart & vessels)
Endoderm = internal structures
e.g. epithelial lining of GI tract, respiratory tract, parenchyma of glands
Are there always three germ layers present?
No - there is no mesoderm layer where the future mouth and anus will form
What is the function of the notochord?
Basis for the axial skeleton (defines the midline)
Sub-specialised part of mesoderm
Formed from prenotochondral cells which migrate through the cranial end of the primitive pit
Drives formation of the nervous system (neuralation)
What is the prochordal plate?
Most cranial part of the notochord
Is the notochord still present in the adult?
Yes - regresses, but part remains in the nucleus pulposus of the intervertebral discs
How are the left and right sides of the embryo differentiated and developed?
Ciliated cells at the primitive node results in the left-ward flow of signalling molecules (only beat in one direction)
This causes side-specific cascades
Left-sided signals = left side
Absence of left-sided signals = right side
Describe situs inversus, its mechanism, and the morbidity of it.
Complete mirror-image of viscera positions e.g. liver on the left instead of on the right
Results from immotile cilia (no signals, therefore random organisation of organs)
Complete mirror image = no associated morbidity
Normal & mirror image = associated morbidity
What are some of the methods of twinning? What is the difference between monozygotic and dizygotic twins?
DICHORIONIC = embryo splits after first cleavage; two embryos formed, each with its own placenta
MONOCHORIONIC = inner cell mass duplicated; two embryos sharing a placenta
MONOCHORIONIC & MONOAMNIOTIC = duplication of primitive streak; two embryos sharing a placenta and amniotic sac (incomplete separation ——–> conjoined twins)
MONOZYGOTIC TWINS: single fertilised oocyte —> identical twins
DIZYGOTIC TWINS: two fertilised oocytes —> non-identical twins
Define teratogenesis. What are some agents that cause teratogenesis?
Normal embryonic development is disrupted
Pre-embryonic = spontaneous abortion
Embryonic = period most sensitive to teratogenic insult
Chemical agents: e.g. thalidomide, alcohol, certain drugs (ACE inhibitors, warfarin, anti-convulsants)
Infectious agents: e.g. rubella
